The policy change was quietly instituted on Feb. 8 after Comcast acquired a controlling interest in NBC Universal, which already had a policy of not accepting ads relating to firearms.

The ban came to light when John Kupiec, president of the advertising agency Canadian American Corp., attempted to purchase an ad for Michigan-based gun store Williams Gun Sight Inc. but was denied, according to CBS News' Detroit affiliate.

"Comcast Spotlight has decided it will not accept new advertising for firearms or weapons moving forward," the cable provider said in a statement to CBS. "This policy aligns us with the guidelines in place at many media organizations."

Executives at Williams Gun Sight called the policy anti-constitutional.

"We’re a perfectly legal company selling a perfectly legal product and they have chosen us out of all the industries out there to make a stand on what’s right or wrong," Williams’ Chief Operating Officer Dan Compeau told CBS.

Kupiec said Comcast is the cable provider for two-thirds of the domestic market, which he believes represents a monopoly in the industry.

"The next step is we want to get the lawmakers on Capitol Hill to review the monopolistic rights this company (Comcast) currently enjoys as the largest cable provider in the United States," Kupiec said, adding that his firm will consider legal action as a last resort.

Compeau said Comcast previously accepted their ads.

"We were totally caught off guard by it," he said. "All these TV stations are taking millions, if not billions, from alcohol companies — and alcohol deaths, alcohol sickness, way outpaces anything a gun can do. [Comcast] is two-faced."

Kupiec, who has worked in advertising since the 1980s, said this is the first major ban he has ever seen, and that if they wanted to advertise on the Outdoor Channel, The Discovery Channel, or NBC Sports, it wouldn't be a possibility.

"If you’re a gun range, if you sell firearms, ammunition, whatever, they will not accept your advertising," he said. "I’m an avid hunter and I believe this is a direct threat on the Second Amendment, a direct assault on legal businesses in the United States, and I think it’s antitrust."